David82:
Are they just connecting two specific wires together when the temp limit is reached? Another way to say it is, if you wanted to make your central HVAC unit run in cool mode and you didn't have the thermostat but did have the wiring for it, how would you make it run?
I've developed a whole-house multiple "Arduino" thermostat system. Yes, you simply complete the circuit to turn the furnace, A/C or fan on. At the same time, you break the circuit to turn each off. It's as simple as that. Dealing with the 24 volt AC power source is more difficult than controlling everything.
Tim